// Assuming m_sMaterial is our material string keyfield
const size_t uMaterialLen = strlen( m_sMaterial );
size_t uDigitLen = 0;
size_t uNextDigit = 1;

for (size_t i = 0; i < NUM_MATERIAL_HANDLES; ++i)
{
    if (i % uNextDigit == 0)
    {
        ++uDigitLen;
        uNextDigit *= 10;
    }

    // Allocate length of material string + length of numeral suffix + null terminator
    // This is non-standard but is used around the engine already so I assume it's compiled with a dynamic stack allocation addon
    const size_t uLength = uMaterialLen + uDigitLen + 1;
    char str[uLength];

    // Copy the base material string
    for (size_t j = 0; j < uMaterialLen; ++j)
        str[j] = m_sMaterial[j];

    // Copy each digit
    size_t uNum = i;

    for (size_t j = uLength - 2; j >= uMaterialLen; --j)
    {
        str[j] = '0' + uNum % 10;
        uNum /= 10;
    }

    str[uLength - 1] = 0;

    m_MaterialHandles[i] = m_ParticleEffect.FindOrAddMaterial(str);
}